The rapid development of electronic devices requires safer batteries having higher energy densities. In this regard, recent research and development efforts to improve battery performance have been mainly focused on lithium and/or Li-ion systems. However, in such battery systems, the conductivity and stability of existing electrolytes have not been optimized with regard to battery safety and performance. In any event, the increased demand for high capacity rechargeable batteries in applications including military devices, electric vehicles (Evs/HEVs) and aero-vehicles has been a principal driving force for the research and development of safe lithium and/or lithium ion batteries.
Lithium and lithium ion batteries normally operate in a voltage range from 3.0 to 4.2 V vs Li/Li+. In liquid or gel-polymer lithium-ion batteries, it is common to use an electrolyte containing alkyl carbonates, such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC) and propylene carbonate (PC). These non-aqueous electrolytes are not thermodynamically stable in the operational voltage range of the batteries. Thus, for such commonly employed electrolytes, it is possible that electrolyte reduction and oxidation could occur at the anode and cathode respectively. In order to prevent such an occurrence, a complete and stable solid electrolyte interphase (SEI) has to be formed. In this regard, it is well known that ethylene carbonate (EC) has excellent SEI-forming properties for carbonaceous anodes, and as a result has been widely used.
Two types of lithium salts, inorganic and organic, are commercially available. Inorganic salts include lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4). Organic salts include lithium trifluoromethane-sulfonate (LiCF3SO3, LiTf), Armand's compounds (see U.S. Pat. No. 4,505,997, the entire content of which is expressly incorporated hereinto by reference), lithium bis(trifluoromethanesulfonyl)-imide (LiN(SO2CF3)2, LiTFSI), and lithium bis(trifluoroethanesulfonyl)imide (LiN(SO2CF2CF3)2, LiBETI). Organic lithium salts are believed to be safer.
Existing salts used in commercial lithium batteries are not as stable or as conductive as expected. In recent years, extensive world-wide efforts have been undertaken to develop practically effective substitutes, especially organic lithium compounds. Cost, performance and safety are considered the three key factors in choosing the electrolyte for commercial applications. All lithium salts including commercially available salts, such as LiPF6, LiBF4, LiOSO2CF3, LiN(SO2CF3)2, LiN(SO2CF2CF3)2, and salts under development, such as LiC(SO2CF3)2, LiBOB, LiPF3(SO2CF3)3, do not fully meet the above three requirements. By way of example, although LiPF6 has excellent conductivity in a liquid electrolyte, a wide electrochemical stability window, and exhibits low toxicity and non-corrosiveness to substrates, it always contains trace amounts of HF acid according to the following equilibrium and hydrolysis as illustrated by Xu et al., Electrochem. Solid-State Letters, 5(1), A26 (2002) (the entire content of which is expressly incorporated hereinto by reference) which degrades battery performance.LiPF6(sol.)⇄LiF(s)+PF5(sol.)LiPF6 (sol.)+H2O→POF3(sol.)+LiF(s)+2HF(sol.)PF5(sol.)+H2O→POF3(sol.)+2HF(sol.)
A process for generating acid-free lithium salt solutions using weak base resins is described in U.S. Pat. No. 6,001,325 (the entire content of which is expressly incorporated hereinto by reference) and seems to work effectively. However, such a process will obviously increase production costs. In order to overcome the problem of generating HF acid, it has been proposed in U.S. Pat. No. 6,210,830 (the entire content of which is expressly incorporated hereinto by reference) to provide LiPF3(CF2CF3)3 to replace LiPF6. When using a LiPF3(CF2CF3)3 containing electrolyte, both the anode (Li or graphite) and cathode (LiMn2O4) performed better as compared with a LiPF6 solution (See, J. Electrochem. Soc., 151, A23 (2004); J. Electrochem. Soc., 150, A445 (2003); and J. Power Sources, 97-98, 557 (2001), each incorporated fully herein by reference). However, LiPF3(CF2CF3)3 is much more expensive than LiPF6.
Other commercially-available salts are problematic also. For example, LiBF4 exhibits poor solubility and has HF contamination. Both LiOSO2CF3 and LiN(SO2CF3)2 are highly corrosive to aluminum substrates at potentials above 2.79 V and 3.67 V respectively, which is not high enough for most advanced rechargeable lithium batteries. Lithium methide, LiC(SO2CF3)2, (see U.S. Pat. No. 5,273,840, the entire content of which is expressly incorporated hereinto by reference) is presently under development, but the price thereof may be an obstacle for consumer applications. The present applicants' testing has shown that, although LiBETI yields performance comparable to that of LiPF6 at ambient temperature, it has poor performance at elevated temperatures (e.g. higher than 80° C.). In any event, the costs for commercially available LiBETI electrolytes is several times higher than those employing LiPF6. A very promising salt, lithium bis(oxalato) borate, LiBOB (German Pat. DE19829030 C1 (1999), incorporated fully by reference herein) also has disadvantages, including unsatisfactory performance in battery systems containing LiCoO2, poor solubility in common carbonate solvents and hydrolytic instability.
Since organic salts may have better safety and higher conductivity, significant efforts have been made to synthesize organic lithium salts which are less expensive and more conductive. Typical examples include lithium borates and phosphates which are well known thermally stable salts with equally well known disadvantages. For example, lithium tetrkis(haloacyloxy) borates, Li[B(OCORX)4] (Yamaguchi, et al., J. Electrochem. Soc., 150, A312 (2003), incorporated fully herein by reference), are less conductive and thermally less stable as compared with LiPF6; lithium bis(polyfluorodiolato) borates, represented by LiB[OCPh(CF3)2]4 (Strauss, et al., J. Electrochem. Soc., 150, A1726 (2003), incorporated fully herein by reference), and have poor solubility in common carbonate solvents. Lithium tris(polyfluorodiolato) phosphates (Nanbu et al., Electrochem. Solid-State Letters, 5(9), A202 (2002) and Eberwein, et al., J. Electrochem. Soc., 150, A994 (2003)) are difficult to prepare and have low oxidative decomposition potential. Lithium salts in which the anions are dicarboxylic acid derivatives of orthoborate are also known. (See, Xu et al, Electrochemical and Solid State Letters, 4(1) E1-E4 (2001), the entire content of which is expressly incorporated hereinto by reference.)
One interesting salt is lithium bis(trifluoroborane)imidazolide [Lilm(BF3)2] (see, Barbarich et al., Electrochem. Solid-State Letters, 6(6), A113 (2003), incorporated fully by reference herein). Using such a compound in lithium batteries was performed by Sun et al., (J. Electrochem. Soc., 149, A355 (2002) and LaPointe et al., (J. Am. Chem. Soc., 122, 9560 (2000), each incorporated fully herein by reference. It has thus been demonstrated that a cell with an electrolyte containing [Lilm(BF3)2] has a comparable performance to a cell containing LiPF6. Unfortunately, the synthesis of such a lithium salt is expensive (i.e., since it requires using n-BuLi) and is time-consuming (i.e., taking more than eight days). In addition, the salt made in such a way contains significant amounts of impurities even after purification. Cells containing such a salt show poor over-change safety and temperature performance. There is no suggestion in the Barbarich et al article of any better synthesis techniques to make [Lilm(BF3)2], nor is there any mention of any type of bridging groups other than imidazolide.
It would thereby be highly desirable if effective and efficient procedures using economical starting materials were provided which enabled the synthesis of a class of organic lithium salts containing extensively charge-delocalized anions, particularly for use in lithium batteries including liquid lithium-ion batteries, gel polymer batteries, dry polymer batteries and solid polymer batteries. It would also be highly desirable the safety performance could be improved by using such prepared salts for lithium batteries and/or increase the energy density by using less content of such prepared salts in the electrolyte. Finally, it would be highly desirable if the fabrication of electrodes and ionic conductive separators could be facilitated using extraction techniques. It is towards providing such needs that the present invention is directed.
Broadly, the present invention is embodied in organic lithium salts and methods of making the same which are usefully employed in electrochemical energy storage systems (e.g., batteries) and exhibit improved properties and performance as compared to conventional lithium salts employed for similar purposes. More specifically, the present invention is embodied in organic lithium salts having the formula Liq[Org(MXn)m], wherein Org represents an organic ion, MXn represents typical inorganic or organic Lewis acids and each of q, n and m is independently 1 or greater, preferably 1 to 4.
Electrochemical cells and batteries, particularly lithium rechargeable batteries, which comprise an anode, a cathode and a non-aqueous electrolytes containing the organic lithium salts of the present invention, exhibit improved properties. For example, the organic lithium salts of the present invention offer enhanced thermal stability, which is crucially required in an extrusion process for making electrodes and ionic conductive separators. Also, the method of making the organic lithium salts of the present invention has an associated lower cost and is simpler (e.g., having one or two reaction steps) as compared to the synthesis procedures of conventional lithium salts.
These and other aspects and advantages of the present invention will become more apparent after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.